The present invention relates generally to all-electronic television receiver tuning systems and, more particularly, to such systems wherein tuning is at least in part accomplished by establishing a "tuning window" about the local oscillator frequency (LOF) corresponding to the picture carrier of a selected television channel.
U.S. Pat. No. 3,961,266 to Tanaka, assigned to the assignee of the present invention, discloses a television receiver tuning system including means for establishing a "tuning window" (i.e. a restricted region of acceptable frequencies) about the LOF corresponding to the assigned picture carrier frequency of a selected FCC designated television channel. Within the tuning frequency window, a conventional automatic frequency control (AFC) system operates to lock the LOF of the tuner in proper relationship to the received picture carrier frequency insuring conversion thereof to the proper IF frequency. Should the LOF drift or otherwise depart from the established boundaries of the tuning window, the tuner initiates a tuning sequence adapted to drive the LOF back within the confines of the window. Thereafter, system tuning is again released to the AFC system. Advantages attributable to a system wherein tuning is initially established relative to a range of frequencies include the capability of properly processing signals subject to carrier frequency errors. Carrier frequency errors of the sort contemplated may result from, for example, broadcast frequency discrepancies as well as discrepancies introduced in signals rebroadcast for cable television systems. The latter, in particular, may suffer from extreme carrier frequency errors deviating by as much as 2MHz from the appropriate FCC allocated carrier frequency. Since a particular picture carrier may be sufficiently offset from its assigned frequency so as to fall outside the boundaries of the tuning window, it would be desirable to provide means conveniently operable for displacing or shifting an established tuning window in frequency space to further increase the tuning system's ability to accommodate such off-frequency picture carriers.
Moreover, in its preferred embodiment, the tuning system described in the foregoing Tanaka patent teaches the use of a .+-.1/2MHz window centered about the LOF corresponding to the assigned picture carrier of a selected television channel. This necessitated a disproportionate AFC range of .+-.1MHz, since there was no way to know in which portion of the frequency window an "off-frequency" carrier might be nor where actual system tuning would be (within the window) when released to AFC.
In order to accommodate picture carriers offset from their FCC - allocated frequency by as much as 2MHz, the previously referenced related copending application describes a tuning system wherein means are provided for initially driving the local oscillator to the frequency corresponding to the assigned picture carrier of a selected television channel prior to establishing the tuning window thereabout. The system, therefore, is always initially tuned to the center of its tuning window and the AFC circuit needs a pull-in range of .+-. one-half the window width. Since conventional AFC systems are generally limited to capture ranges of .+-.2MHz, it would therefore be advantageous to provide means in association with the tuning system for establishing a tuning window consisting of a 4MHz range of frequencies centered about the LOF corresponding to an assigned picture carrier.